The present invention relates to multicasting technology for delivering information, such as a data content.
Multicasting is a known communication technique that is effective for delivering the same information (such as content) to multiple (user) terminals. Multicasting is advantageous over unicasting, i.e., point-to-point transmission of information, because it reduces the load on a delivery server and reduces traffic. In multicasting, an Internet Group Membership Protocol (IGMP) may be used for IPv4 and a Multicast Listener Discovery (MLD) protocol may used as a protocol for IPv6. The IGMP is defined in the Request for Comments (RFC) 1112 and RFC 2236 documents, which are Internet standards laid open by the Internet Engineering Task Force (IETF). Likewise, the MLD protocol is defined in the RFC 2710 document.
The above-mentioned IGMP and MLD protocols may be used between hosts and packet routing devices (such as gateways and routers). In multicasting, for effective delivery of the same data to multiple hosts, these protocols are used to control host groups (multicast groups) configured for receiving multicast datagrams. The IGMP and MLD protocols are used when a host sends a request to join a multicast group (request for delivery of multicast data) and when a host sends a request to leave a multicast group (request to stop delivery of multicast data).
According to the above-described multicasting technique, for example, when a host wishes to join or leave a particular multicast group, it sends an appropriate IGMP or MLD message to a packet routing device. The packet routing device can determine all hosts (registered member hosts) belonging to each of the multicast groups under its management by IGMP or MLD. Therefore, the packet routing device can transmit multicast data efficiently to such member hosts by making as many copies of one multicast packet as the number of specified hosts and transmit the copies simultaneously to all of the hosts.
Meanwhile, it is possible that a user of an unregistered host may try to access a multicast group improperly. In this case, the unregistered host transmits, to a packet routing device, a request packet to join the multicast group. However, in the conventional system, the packet routing device rejects the request to join received from the unregistered host and just discards the received packet, thus sending no response to the unregistered host. As a result, the unregistered host cannot recognize why it cannot join the multicast group in response to its request. If a reply were to be sent to the unregistered host, then data delivery refusal messages would be sent to all hosts belonging to the multicast group (which is the object of the request to join the group).
Content delivery service providers may wish to provide a preview or digest of a data content to be delivered to unregistered hosts and also to solicit membership registration with a multicast group. However, packet routing devices having the above-described background arrangements have no way of sending responses exclusively to unregistered hosts, as described above.